In general, a braking system that brakes a running vehicle using a hydraulic pressure for increasing a driver's control force of a brake pedal adopts a simple method that holds rotating wheels to stop the rotation. In this case, however, it is difficult to implement optimum braking performance according to running states of the vehicle and road conditions.
Accordingly, there are used various technologies and methods that control a braking liquid pressure at the time of braking in order to overcome the limitation of the simple braking system and to improve safety of the vehicle.
For example, the following systems can be applied: ABS (Antilock Brake System) that appropriately adjusts a braking pressure to be applied to wheels according to a slip ratio calculated from a wheel speed, thereby preventing locking of the wheels; TCS (Traction Control System) that adjusts a driving force of an engine in order to prevent an excessive slip at the time of sudden start or rapid acceleration of the vehicle; and ESP (Electronic Stability Program) that minimizes a difference between a desired vehicle running direction and an actual vehicle running direction and keeps a vehicle running direction according to a driver's intension under any driving conditions.
Such a braking pressure control is implemented by providing various solenoid valves on a hydraulic circuit formed between a wheel cylinder for holding and restricting a disk wheel and a master cylinder for generating a hydraulic pressure, and controlling a path from the master cylinder to the wheel cylinder and a return path from the wheel cylinder to master cylinder.
At this time, a controller (typically, an ECU (Engine Control Unit) is applied) is used for liquid pressure control on the hydraulic circuit, in addition to control of electrical components for implementing a braking operation. That is, the controller reads the wheel speed and the like, then performs arithmetic, analysis, and judgment according to an internal control program, and subsequently performs the pressure control on a hydraulic line by driving an oil pump or the like, in addition to on/off control of the solenoid valves.
However, with the simple on/off control of the solenoid valves, at the time of implementing the ABS or the TCS, there is a limitation in that a braking pressure cannot be controlled to have a desired pressure gradient. For this reason, in order to implement precise liquid pressure control, there is needed a technology that controls the solenoid valves in a PWM (Pulse Width Modulation) method, and implements a braking pressure to have a pressure gradient according to a braking state of a vehicle at the time of braking.
Meanwhile, in the PWM control, known on/off type solenoid valves cannot be applied, and thus it is necessary to separately develop solenoid valves that can be controlled to perform a desired operation in the PWM method. For this reason, costs for development and mass, production may be required. Then, upon application, competitiveness may be weakened in terms of the costs.
In addition, at the time of normal braking, that is, when the ABS or the TCS does not operate, there is a phenomenon that a difference in pressure intensification gradient occurs due to a difference in generation time of a braking pressure between front and rear wheels. In this case, even though PWM control type solenoid valves are applied to the braking hydraulic circuit, the above phenomenon is not solved. Accordingly, there is a limitation in that unstability at the time of braking cannot be thoroughly solved.